1. Field of the Invention
The present invention relates generally to a method and apparatus for multiple mode imaging, and more particularly to catadioptric optical systems used for dark field imaging applications.
2. Description of the Related Art
High precision optical instruments and imaging systems used in many different applications must operate effectively and efficiently. To accommodate optical functionality under varied conditions, precision lenses are often employed in different complex combinations.
Many different imaging modes exist for optical inspection. These imaging modes include bright field, confocal, and a variety of dark field imaging modes. Typically each different mode requires a different machine. Full inspection of an object, such as a semiconductor wafer, requires several separate very expensive machines. Combining -many different imaging modes into one machine can dramatically reduce inspection costs as well as provide performance advantages.
The bright field imaging mode is commonly used in microscope systems. The advantage of bright field imaging is the image produced is readily distinguishable. The size of image features accurately represents the size of object features multiplied by the magnification of the optical system. This technique can be more easily used with image comparison and processing algorithms for computerized object detection and classification.
The confocal imaging mode has been successfully used for optical sectioning to resolve the height differences of object features. Most imaging modes have difficulty detecting changes in the height of features. The confocal mode forms separate images of object features at each height of interest. Comparison of the images then shows the relative heights of different features.
The dark field imaging mode has been successfully used to detect features on objects. The advantage of dark field imaging is that flat specular areas scatter very little light toward the detector, resulting in a dark image. Any surface features or objects protruding above the object scatter light toward the detector. Thus, in inspecting objects like semiconductor wafers, dark field imaging produces an image of features, particles, or other irregularities on a dark background.
Dark field illumination provides a large signal for small features that scatter light. This large signal allows larger pixels to be used for a given feature size, permitting faster object inspections. Fourier filtering can also be used to minimize the repeating pattern signal and enhance the defect signal to noise ratio.
Each dark field mode consists of a specific illumination scheme and collection scheme such that the scattered and diffracted light collected from the object provides the best signal. Several optical systems have been developed that use different dark field imaging modes including laser directional dark field, double dark field, and central dark ground.
One prior method for achieving laser directional dark field imaging is disclosed in U.S. Pat. No. 5,177,559, issued Jan. 5, 1993 to Batchelder and Taubenblatt and assigned to International Business Machines, which is hereby incorporated by reference. This method uses a collimated beam of monochromatic light to illuminate a semiconductor wafer from outside the objective between an angle of 8 degrees from the horizontal and the numerical aperture, or NA, defined by the imaging objective. Before forming a dark field image, the collected light passes through a Fourier filter to attenuate the spatial frequency components corresponding to repeating array patterns.
This laser directional dark field method illuminates the wafer outside the NA of the imaging objective. For this reason, the illumination angles are limited to between 8 degrees from the horizontal and the NA defined by the imaging objective. Collection angles are also limited to the range of angles within the NA of the objective. A long working distance objective is necessary to allow access by the laser to the area of interest on the semiconductor wafer. Objectives used in dark field applications of this type are generally limited to NAs less than 0.7, which corresponds to collection angles of only up to 44 degrees from normal.
Another prior method for achieving laser directional dark field imaging is disclosed in U.S. Pat. No. 5,428,442, issued Jun. 27, 1995 to Lin and Scheff and assigned Optical Specialties, which is hereby incorporated by reference. This method uses a collimated beam of monochromatic light illuminating the wafer from inside the optical system, within the NA defined by the objective. If the system will encounter a specific range of defect sizes, the illumination angle on the wafer is chosen so the optical system collects spatial frequencies of interest.
This is a laser directional dark field method wherein the laser illuminates the wafer from inside the NA as defined by the objective. The system uses the same objective pupil plane for injecting the illumination and processing the light collected by the objective. This objective pupil feature seriously limits the types of illumination and Fourier filtering that are possible. Systems using objectives of this type are generally limited to NAs of less than approximately 0.9. This means illumination angles are limited to less than approximately 64 degrees. Illumination at angles above 64 degrees is often necessary to obtain optimum defect sensitivity. This high angle illumination is not possible without a higher NA objective. The available objectives of this type with a high NA have very small fields, relative to that of a lower NA objective. This seriously limits the number of resolvable points in the image and the achievable inspection speed. Another problem with this technique is small amounts of scattered and reflected light from lens elements in this system have the ability to produce noise at levels that compromise the sensitivity. Introducing laser illumination from inside this type of objective can cause a significant amount of scattered and reflected light from the multiple lens surfaces. The system must deal with scattered light from the lenses, illumination beam and the specular reflection from the wafer, which is a tremendous potential problem.
A third known dark field imaging method designed to detect particles on a periodic patterned object is disclosed in U.S. Pat. No. 4,898,471, issued Feb. 6, 1990 to Stonestrom et al. and assigned to Tencor Instruments, which is hereby incorporated by reference. This method uses a single light beam scanned at a shallow angle over the object. The position of the collection system as well as the polarization of the light beam may be arranged to maximize the particle signal compared to the patterned signal.
This single beam/shallow angle system uses an off axis collector to image the area of interest onto a detector. The position of the collection system as well as the polarization of the illumination is arranged to maximize the signal scattered by particles. Only a single angular position is used for illumination and another angular position for collection. Such a dark field system uses a single spot to scan across the wafer in conjunction with a single detector. If the system uses a small spot for high sensitivity detection, the inspection speed tends to decrease dramatically. If a larger spot size is used to increase inspection speed, the overall system sensitivity degrades.
In the practical industrial application for object inspection the scattered and diffracted light is collected from either side of the plane of incidence. Since this dark field mode collects light outside of the plane of incidence, this mode is categorized as double dark field. The double dark field technique often obtains maximum sensitivity when the collection angle is greater than 70 degrees from normal, which is well outside the range of a 0.9 NA objective. This makes the combination of the double dark field mode and other imaging modes such as bright field and laser directional dark field difficult.
Three physical embodiments for performing dark field imaging are presented in FIGS. 1-3. FIG. 1 presents a directional dark field system 100 which illuminates the object 101 using a laser beam 102 directed at a high angle of incidence. Light contacting an anomaly is scattered or diffracted upward through collector 103, lens 104, and finally to the image plane 105.
FIG. 2a is side view of a double dark field design system 200 which illuminates the object 201 using laser beam 202 directed at a relatively low angle of incidence. Collectors 203 and 204 are mounted at different angles from the laser beam 202, typically 90 degrees. FIG. 2b illustrates a top view of the system with the collectors 203 and 204 mounted 180 degrees from one another and 90 degrees from the laser 202. Variations of these angles are possible. This provides enhanced collection capability and allows detection of particular object faults.
FIG. 3a illustrates a variation of a central dark ground imaging system 300, wherein the laser beam 301 passes through the collector 302 at an approximately perpendicular angle to the object 303. The light beam strikes the object and is diverted, depending upon the features encountered, toward various collectors mounted about the object. Four collectors 305-308 in FIG. 3b have been employed in the past, each at an angle 90 degrees from the nearest collectors, as shown in FIG. 3b. Different numbers of collectors may be used at various angles depending upon the type of object scanned and the defects anticipated.
The drawbacks of these prior dark field systems are twofold. First, physical limitations tend to restrict the angle at which light beams may be applied to the object, i.e. at either extremely deep or extremely shallow angles. This angular limitation tends to impede a full examination of the object. For example, if a type of defect may be detected best when light is applied at a 45 degree angle, such defects may not be apparent when the illumination source is limited to an angle less than 45 degrees. Secondly, these systems only employ one dark field mode each, which again tends to limit the types of defects which may be detected. Any attempt to combine different dark field imaging systems would be severely inhibited due to physical component placement restrictions and the inherent cost associated with multiple components.
With respect to the use of a single imaging mode, different schemes have varying performance advantages. For example, a double dark field arrangement is useful when small particles exist on the object, or rough films are expected to be encountered. Microscratches are best detected using normal incidence illumination and off normal imaging. Grazing incidence and back scattering may be used to detect missing contacts or vias on semiconductor wafers, while full sky mode best address metal grain variations. As the types of defects which may be encountered vary depending on many manufacturing factors, it would be best to inspect object for all of these defects. The problem, however, is that no single mode provides these capabilities, nor is a combination of these modes physically realizable nor economically feasible. Further, the use of multiple arrangements running over different machines negatively effects the scanning, processing, and evaluating time for each specimen.
It is therefore an object of the present invention to provide an apparatus that can incorporate as many imaging modes as possible and still maintain image quality and throughput requirements. Such an apparatus would be based on a high NA catadioptric objective with highly flexible illumination and imaging capabilities. Several prior catadioptric systems have been developed for microscopic inspection. While these systems may have a similar appearance to the present invention, they simply do not have the capability to integrate multiple inspection modes.
One apparatus previously used for microscopic inspection is the catadioptric bright field imaging system disclosed in U.S. Pat. No. 5,031,976, issued Jul. 16, 1991 to Shafer and assigned to KLA Instruments, which is hereby incorporated by reference. The system employs a focusing lens, a field lens, and a spherical mirror to focus and reflect light toward an object and back through an aperture located in the spherical mirror. The broadband aspects of the system disclosed do not directly translate to dark field imaging, but the patent does present an apparatus capable of 0.6 NA broad-band UV imaging for the deep ultraviolet spectral region, i.e. approximately 0.19 to 0.30 micron wavelengths.
The ""976 patent is based on the Schupmann achromatic lens principle, which produces an achromatic virtual image, wherein an achromatic real image is produced from the virtual image using a reflective relay. The lens system is formed from a single glass type, fused silica. A series of lenses and mirrors is used to correct for aberrations and focus the light.
The system used in the ""976 patent is shown in FIG. 4. The system includes an aberration corrector lens group 401 for correcting image aberrations and chromatic variation of the image aberrations, a focusing lens 403 for receiving light from he group 401 and producing an intermediate image plane at plane 405, a field lens 407 of the same material as the other lenses placed at the intermediate image plane 405, a thick lens 409 with a plane mirror back coating 411 whose power and position are selected to correct the primary longitudinal color of the system in conjunction with the focusing lens 403, and a spherical mirror 413 located between the intermediate image plane and the thick lens 409 for producing a final image 415. Most of the focusing power of the system is due to the spherical mirror 413 which has a small central hole near the intermediate image plane 405 to allow light from the intermediate image plane 405 to pass through to the thick lens 409. The mirror coating 411 on the back of the thick lens 409 also has a small central hole 419 to allow light focused by the spherical mirror 413 to pass through to the final image 415.
The ""976 system is a catadioptric objective capable of 0.6 NA, with broad-band UV imaging over a 0.5 mm field. This limited NA and field size does not support high speed inspection with more than one dark field mode. An NA of 0.6 collects only 20% of the available solid angle above the sample object. The maximum field size for this system is 0.5 mm. This limits its maximum pixel size and data collection speed. For example, a system with 0.5 mm field size would only have 25% of the maximum pixel size and data collection speed compared to a system with a 2 mm field size. The thickness of the lens mirror element in the ""976 patent prevents the system from being scaled up for larger field sizes. The objective is formed from fused silica and uses a positive fused silica field lens near the intermediate image to correct for low order lateral color. The thick lens mirror element is also used in the catadioptric group to compensate the chromatic variation in aberrations of the external lens group. This broad band objective also has no provision for laser illumination through the catadioptric elements.
U.S. Pat. No. 5,488,229 to Elliott and Shafer and the associated continuation in part application, both of which are hereby incorporated by reference, provide a modified version of the optical system of the ""976 patent, one which has been optimized for use in 0.193 micron wavelength high power excimer laser applications such as ablation of a surface 421xe2x80x2 as seen in FIG. 5. This system has an aberration corrector group 401xe2x80x2, focusing lens 403xe2x80x2, intermediate focus 405xe2x80x2, field lens 407xe2x80x2 thick lens 409xe2x80x2, mirror surfaces 411xe2x80x2 and 413xe2x80x2 with small central opening 417xe2x80x2 and 419xe2x80x2 therein and a final focus 415xe2x80x2 as in the prior ""976 patent, but repositions the field lens 407xe2x80x2 so that the intermediate image or focus 405xe2x80x2 lies outside of the field lens 407xe2x80x2 to avoid thermal damage from the high power densities produced by focusing the excimer laser light. Further, both mirror surfaces 411xe2x80x2 and 413xe2x80x2 are formed on lens elements 40xe2x80x2 and 409xe2x80x2. The combination of all light passing through both lens elements 408xe2x80x2 and 409xe2x80x2 provides primary longitudinal color correction of the single thick lens 409 in FIG. 4, but with a reduction in total glass thickness. Since even fused silica begins to have absorption problems at the very short 0.193 micron wavelength, the thickness reduction is advantageous at this wavelength for high power levels. Though the excimer laser source used for this optical system has a relatively narrow spectral line width, the dispersion of silica near the 0.193 micron wavelength is great enough that some color correction is still needed. Again, the system has a numerical aperture of about 0.6.
U.S. Pat. No. 5,717,518 to Shafer, Chuang, and Tsai and the associated continuation-in-part application describe a catadioptric objective capable of high NA, ultra broad-band UV bright field imaging. This is a 0.9 NA system with very tight manufacturing tolerances, primarily due to the broad-band requirements of the design. The broad-band UV nature of the design is due in part to the use of two glass types, calcium fluoride and fused silica. Calcium fluoride has excellent UV transmission properties but it is not necessarily a desirable glass because it is difficult to polish, not mechanically stable, and expensive. Fused silica is a more desirable material because in addition to its good UV transmission it has desirable manufacturing properties.
The system in the ""518 patent is shown in FIG. 6. The focusing lens group 11 consists of seven elements 21-27 with two of the lenses 21 and 22 separated from the remaining lens elements. These two lenses form a nearly zero power doublet for the correction of chromatic variation of monochromatic imaging aberrations. The five lenses 23-27 form the main focusing subgroup. The subgroup focuses the light to an intermediate image 13. The curvature and positions of the lenses in this subgroup are selected to minimize the monochromatic aberrations as well as cooperate with the doublet 21-22 to minimize chromatic variations of those aberrations. The field lens group 15 typically composes an achromatic triplet. Both fused silica and calcium fluoride glass materials are used. The catadioptric group 17 includes a meniscus lens 39 witha concave reflective surface coating 41. It also includes a second optical element consisting of a lens element 43 with a reflective surface coating 45 on a back surface of the lens. The first lens 39 has a hole 37 centrally formed along the axis of the optical system. Light from the intermediate image 13 passes through the optical aperture 37 in the first lens 39 and then throughout the body of the second lens 43 where it is reflected back by mirror surface 45. The light then returns through the body of 43 and the body of 39 to reflect off surface 41. The light, now strongly convergent, passes through the body of 43 and an aperture 47 in the reflective surface coating to firm the final image 47.
Common problems with using several of these prior catadioptric optical systems for multiple imaging modes includes: these are complicated systems with tight manufacturing tolerances, they provide no illumination and imaging above 0.9 NA and a field of view less than 1 mm, pupil planes that are not easily accessible, and limited imaging modes and associated illumination schemes.
It is therefore an object of the present invention to provide a method and an apparatus which can incorporate as many imaging modes as possible while still meeting image quality and throughput requirements.
It is another object of the current invention to provide an effective, relatively simple and low cost imaging system having high optical performance and minimal inspection times.
It is a further object of the current invention to provide a high numerical aperture imaging system having high optical performance.
It is yet another object of the current invention to provide a system having a pupil plane at a location separate and apart from the lenses of the optical system, and providing the ability to magnify the resultant image over a range of magnification values.
The present invention is a multiple mode catadioptric imaging method and apparatus for optical inspection. The inventive apparatus disclosed herein can combine the functions of several prior imaging modes and is based on a high NA catadioptric optical design. The system can be a narrow band system preferably having an NA greater than 0.90 that is highly corrected for low and high order monochromatic aberrations.
The high NA catadioptric design disclosed herein may be used and optimized for light having different wavelengths, from infrared to the deep ultraviolet. For example, in the ultraviolet spectrum, light beams having wavelengths of 193 nm, 213 nm, 244 nm, 248 nm, 257 nm, 266 nm, and so forth are possible using the concepts disclosed herein, with adjustments that would be apparent to those of ordinary skill in the art.
An example of an inventive apparatus that supports multiple mode imaging is disclosed. This apparatus is a 0.97 NA catadioptric optical design having a 1 millimeter field size is that optimized for a 266 nm wavelength and uses only fused silica. The design includes a catadioptric group and a focusing optics group proximate to an intermediate image. Light scattered, diffracted, and reflected by the object is collected by the catadioptric group which forms an intermediate image. The focusing optics group then corrects the aberrations present in the intermediate image. The working distance of the design presented is 0.5 millimeters. The central obscuration is less than to 10% of the pupil diameter, and the system can have a Strehl ratio of better than 0.98 over the entire field.
The catadioptric group includes a fused silica single near flat reflector and a dome-shaped reflector arranged in a catadioptric group. The near flat reflector can be a parallel fused silica plate having zero power, thereby making the catadioptric group easy to align. The focusing optics group includes eight lens elements in one embodiment. The focusing optics group corrects for high order spherical aberration and coma.
The ultra high NA disclosed allows for a variety of flexible illumination schemes. Illumination angles from 0 to approximately 85 degrees can be easily implemented thereby allowing maximum flexibility when choosing an illumination angle.
The catadioptric optical system has two primary methods for illumination. First, light energy can enter through the lenses of the optical system at angles between 0 and the collection angle defined by the objective NA. Second, for oblique illumination at angles from about 12 to 85 degrees, the preferred method of is by introducing the illumination through an aperture in the coating of the dome-shaped reflector. Light energy may illuminate the object at angles from 12 to 85 degrees from the normal. With proper mirror design, a beam introduced through an aperture in the mirror coating maintains the same angle with respect to the wafer when it exits the near flat reflector. In addition, the catadioptric group will not add any power to the illumination beam. The aperture required in the mirror coating of the catadioptric group may be a slit of non-mirrored surface where light energy is applied.
Multiple beams at multiple angles may also be used for illumination. For example, two beams separated azimuthally by 90 degrees may be used for illumination to minimize shadowing effects, with appropriate apertures provided.
The catadioptric imaging system effectively collects light scattered, diffracted, and reflected at different angles by the wafer and maps these scattering angles to a plane. This plane is located at the pupil of the system and each position on this pupil plane corresponds to a position on dome-shaped reflector. Each location in the pupil plane corresponds to different scattering angles, and apertures placed at this pupil plane can be used to limit the range of scattering angles reaching the image detector. This pupil plane roughly corresponds to the Fourier plane of the wafer. Such a system supports collection NAs up to 0.99
The wide range of illumination and collection angles possible with the system supports multiple imaging modes. These modes include but are not limited to: variable NA bright field, full sky, ring dark field, inverted ring dark field, directional dark field, double dark field, central dark ground, Manhattan geometry, confocal bright field, confocal dark field, and well as conoscopic imaging. Other schemes are possible in which the illumination angle is between 0 and approximately 85 degrees and the collection angle between 0 and approximately 82 degrees.
The variable NA bright field imaging mode may be employed using the apparatus disclosed herein. The illumination of the wafer in this system is similar to that in a standard microscope. The light can be injected into the optical system using a beam splitter and then projected through the optical system to the object to be inspected. Variable NA illumination can be obtained by using an aperture at the objective pupil, in the collimated range of the objective, or in separate optics before the beam splitter. The light reflected, scattered, and diffracted from the object returns through the objective and through the beam splitter. Variable NA of the light from the object can be obtained by using an aperture at the objective pupil or in the collimated range of the objective. An image is formed on a detector using additional lenses as described in the embodiments below. Such a bright field imaging technique can also operate in full sky mode. In this mode the image contains light scattered at angles up to the maximum possible NA of the objective. The full sky mode is most effective when the collection NA is near 0.99.
Alternately, the apparatus disclosed herein can operate in ring dark field imaging mode. This is a standard type of dark field imaging where the illumination angles are limited to a high NA ring and the imaging angles are limited to the NAs less than those used by the ring illumination. The ring illumination can be injected into the optical system using a beam splitter. The low NA imaging can be obtained by placing an aperture at the pupil plane of the objective to limit the angles reaching the detector. This system can also operate in inverted ring dark field mode. This mode is the inverse of the previous system and uses low NA bright field illumination and a high NA ring for imaging.
The apparatus disclosed herein can operate in directional dark field imaging mode. This mode uses oblique illumination as described above and can collect scattered light from the object up to the full angular range of the catadioptric objective. Collecting only the desired portion of the scattered and diffracted light is accomplished by using an aperture in the pupil plane or in the collimated range of the objective.
Alternately, the apparatus disclosed herein can operate in the double dark field imaging mode. This mode uses oblique illumination and collects the near 90 degree azimuthal portions of the scattered and diffracted light. The system uses an aperture in the pupil plane or in the collimated range to limit the collection to the near 90 degree azimuthal portions of scattered and diffracted light.
Additionally, the apparatus disclosed herein can operate in the central dark ground imaging mode. This mode uses normal illumination by directing light through the optical system at an angle from approximately 0 to 12 degrees from the normal of object being inspected. In this mode the specular reflection from the wafer is blocked and remaining portions of the scattered and diffracted light are transferred to the detector.
The apparatus disclosed herein can also operate in the Manhattan geometry imaging mode. This mode can use normal illumination as described in the central dark ground mode or oblique illumination as described in the directional dark field mode. The Manhattan geometry uses high angle light collection from four different quadrants.
Additionally, the inventive system disclosed herein can operate in the bright field confocal imaging mode. This mode takes advantage of the short depth-of-focus obtainable by using a high NA objective and short wavelength illumination. Bright field confocal mode illuminates the object with a single point or a line focus. The illumination spot on the object is then imaged through an aperture in front of a detector. The object, illumination spot, aperture, or a combination thereof are then scanned to collect information about an area on the object being examined.
Additionally, the system can operate in the dark field confocal imaging mode. The dark field confocal imaging mode also takes advantage of the short depth-of-focus obtainable by using a high NA objective and short wavelength -illumination. This unique imaging mode is made possible by the high NA diffraction limited illumination. High NA ring illumination produces a diffraction limited spot or line and the remaining NA can support diffraction limited imaging. The illumination spot on the object is imaged, using an NA that is less than the illumination NA, through an aperture in front of a detector. The object, illumination spot, aperture, or a combination thereof are then scanned to collect information about an area on the object being examined.
Additionally, the apparatus disclosed herein can operate in the conoscopic imaging mode. This mode can use either oblique or normal illumination, and lenses are not required to form an image on a detector. The light at the pupil plane or in the collimated range of the objective can be placed directly on a single detector, multiple detectors, or detector array.
As may be appreciated from the previous paragraphs, the inventive concept disclosed herein is that multiple imaging modes can be implemented using a single ultra high NA optical system or machine. This is possible because of the wide range of illumination and imaging angles supported by this optical system. Illumination angles from 0 to 85 degrees from normal can be easily implemented, thereby allowing maximum flexibility when choosing an illumination angle. Collection angles from 0 to 82 degrees from normal are also possible. Different illumination schemes and different pupil filters are used to select the different imaging modes.
The narrow band catadioptric optical system disclosed herein may be designed with NAs up to 0.99 in air. The system can also be designed with field sizes of greater than 2 mm. Such a system has relaxed manufacturing tolerances and only requires a single glass material. One embodiment of the system disclosed herein provides increased field size, including up to 4.0 mm of field size.
A further embodiment of the current design provides a relayed pupil for better access to the Fourier plane. Transferring the pupil to an external plane affords easy simultaneous operation of various imaging schemes.
Additionally, a tube lens group can be used with any of these objective designs to provide a magnified image of the object being inspected. Different tube lens groups may be used to obtain different magnifications. The tube lens groups can be designed to have a distant exterior pupil plane to match the buried interior pupil plane of the catadioptric objectives. A tube lens group can also be designed to work with an objective with a relayed pupil.
Still another embodiment of the current system is a six element varifocal tube lens group which can be independently corrected from the main system and which provides magnification of the image from 20xc3x97 to 200xc3x97.
These and other objects and advantages of all of the aspects of the present invention will become apparent to those skilled in the art after having read the following detailed disclosure of the preferred embodiments illustrated in the following drawings.